Cerium/zirconium mixed oxide catalysts having high/stable specific surface areas

ABSTRACT

Cerium/zirconium mixed oxides (optionally including yttrium values), comprising solid solutions thereof, having contents of zirconium of up to 60 atom % and having thermally stable, very high specific surface areas at least greater than 80 m 2  /g, preferably at least 100 m 2  /g and more preferably at least 150 m 2  /g, are well suited as catalysts and/or catalyst supports, notably for the treatment/conversion of vehicular exhaust gases; such Ce x  Zr 1-x  O 2  particulates are conveniently prepared by thermally treating an aqueous solution of soluble compounds of cerium and zirconium (and optionally yttrium), e.g. the nitrates thereof, present in the desired stoichiometric amounts, and thence recovering and, if appropriate, calcining the reaction product thus formed.

This application is a continuation of application Ser. No. 08/170,725,filed Dec. 21, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel compositions based on mixedoxides of cerium and zirconium, and optionally yttrium, especiallyhaving improved specific surface areas, in particular high and thermallystable specific surface areas.

This invention also relates to a process for the preparation of suchcompositions, and to the use thereof, notably in the field of catalysis,whether as catalysts, per se, and/or as catalyst supports.

2. Description of the Prior Art

Cerium oxide and zirconium oxide are known compounds that areparticularly useful constituents, either alone or in combination, in awide variety of catalyst compositions, e.g., multifunctional catalystcompositions, especially catalysts suited for the treatment orconversion of exhaust gases emanating from internal combustion engines.By "multifunctional" is intended a catalyst capable of effecting notonly the oxidation, in particular, of carbon monoxide and ofhydrocarbons present in the exhaust gas, but also the reduction of theoxides of nitrogen also present in such gas ("three-way" catalysts).

It will be appreciated that such catalysts, both at the level of theircompositional nature, as well as their principle of action, are widelydescribed in the literature, both patent and otherwise. Given that thescientific theories which to date have been advanced to explain thisfact still appear somewhat doubtful, and at times even contradictory, itnevertheless now appears well established that the "three-way"industrial catalysts, at the same time containing both cerium oxide andzirconium oxide, are overall more effective than those catalysts whichare either totally devoid of the aforesaid two oxides, or devoid of onlyone of them.

In catalysts such as those indicated above, the cerium oxide and thezirconium oxide, which moreover can exert a proper catalytic functionand/or a simple support function for other catalytic elements such asplatinum, rhodium and other precious metals, are generally present in anuncombined form, namely, these two constituents are present in the finalcatalyst in the form of a simple physical admixture ofwell-individualized oxide particles. This results in part from the factthat these catalysts based on cerium oxide and zirconium oxide arecharacteristically produced by intimate mixing of the correspondingoxide powders, or even of thermally decomposable precursors of theseoxides.

However, for a variety of reasons, a more and more marked tendency isdeveloping in this art to introduce and to employ the elements ceriumand zirconium in the catalyst composition, not in a separate anduncombined form, but, to the contrary, directly in the form of a truemixed oxide CeO₂ /ZrO₂ of the solid solution type.

Nonetheless, in such catalysts a mixed oxide is required having aspecific surface area which is as high as possible and also, preferably,thermally stable. Indeed, taking account of the fact that the efficiencyof a catalyst is generally all the greater when the surface area ofcontact between the catalyst (catalytically active phase) and thereactants is high, it is expedient that the catalysts, both while freshand after prolonged use at more or less elevated temperatures, bemaintained in a state which is the most finely divided possible, i.e.,the solid particles, or crystallites, comprising same should remain assmall and as individualized as possible. This cannot be attained exceptby starting from mixed oxides having high specific surface areas andwhich are relatively stable to temperature.

It too will be appreciated that certain mixed oxides of the solidsolution type in the system CeO₂ /ZrO₂ are known to this art.Nonetheless, their preparation generally requires a calcination stage atrelatively high temperature to obtain a single cubic phase. Compare, forexample, E. Tani, M. Yoshimura and S. Somiya, "Revised Phase Diagram ofthe System ZrO₂ --CeO₂ below 1400° C.", published in J. Am. Ceram. Soc.,vol. 66 7!, pp 506-510 (1983). The phase diagram illustrated in thispublication thus indicates that, to obtain a stable phase crystallizingin the cubic habit, it is necessary to conduct calcinations and/orthermal treatments at temperatures at least greater than 1,000° C. Butthis is of course incompatible with producing a mixed oxide of highspecific surface area. Indeed, at such elevated calcinationtemperatures, where the desired solid solution is certainly formed, thespecific surface area of the product obtained does not exceed 10 m² /g,and typically is even less than 5 m² /g. In summary, the mixed oxidesheretofore known to this art are therefore not well suited for catalystapplications.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofnovel CeO₂ /ZrO₂ mixed oxides of the solid solution type, having largespecific surface areas, and this over a wide range of compositions, inparticular at high contents of zirconium.

Another object of the present invention is the provision of such novelCeO₂ /ZrO₂ mixed oxides which retain a large specific surface area evenafter calcination(s) at elevated temperatures.

Still another object of this invention is the provision of particularsynthetic technique for the preparation of said novel CeO₂ /ZrO₂ mixedoxides.

Briefly, the present invention features particulates of cerium/zirconium(and optionally yttrium) mixed oxides, comprising solid solutionsthereof and having a specific surface area of at least 80 m² /g,prepared by thermally treating/reacting an aqueous formulation ofwater-soluble compounds of cerium and zirconium (and optionallyyttrium), said compounds of cerium, zirconium and, optionally, yttriumbeing present in predetermined stoichiometric amounts, and thencerecovering the reaction product thus produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 6 are X-ray diffraction spectra of cerium/zirconium mixedoxides according to the present invention, as well as of comparativesamples.

DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENTS OF THEINVENTION

More particularly according to the present invention, it has nowsurprisingly been found that true solid solutions between a cerium oxideand a zirconium oxide (and optionally an yttrium oxide) can be producedvia technique, one of the critical and principal features of which isthat it is carried out at reaction temperatures which are so unusuallylow vis-a-vis the known state of the art of the synthesis of solidsolutions. The products thus formed then naturally have specific surfaceareas which are sufficiently high for catalyst applications.

Herein, by "specific surface area" is intended the B.E.T. specificsurface area determined by adsorption of nitrogen in accordance with thestandard ASTM D 3663-78, emanating from the BRUNAUER-EMMETT-TELLERtechnique described in Journal of the American Chemical Society, 60, 309(1938).

Also, each time that the expression "mixed oxide based on cerium andzirconium" is employed, it connotes a composition which can additionallycontain yttrium, in solid solution in the cerium oxide.

The present invention thus features novel compositions based on mixedoxides of cerium, zirconium and, optionally, of yttrium, saidcompositions being characterized in that they have a specific surfacearea of at least 80 m² /g.

This invention also features a process for the preparation of said novelcompositions, comprising the following essential stages:

(i) first providing a mixture, in aqueous solution, in the requiredstoichiometric proportions, of soluble compounds of cerium and ofzirconium, and optionally of yttrium,

(ii) heating the mixture thus formulated,

(iii) recovering the reaction product thus obtained, and, ifappropriate,

(iv) calcining the reaction product thus recovered.

The process according to the invention permits obtaining pure phases ofthe mixed oxide type at synthesis temperatures as low as about 100° C.The phases thus formed are clearly shown by means of X-ray diffractionanalyses conducted on the products later calcined at about 400° C. Thecalcination stage, therefore, essentially enables the crystallinity ofthe solid solutions to be developed and/or their specific surface areato be adjusted to a final desired value for a given application.

The compositions according to the invention are primarily characterizedby their extremely high specific surface area, namely, higher than 80 m²/g.

Advantageously, the compositions according to the invention have aspecific surface area of at least 100 m² /g, more preferably of at least140 m² /g, and even more preferably of at least 150 m² /g.

In addition, according to another characteristic of the compositions ofthe invention, when these are subjected to relatively high calcinationtemperatures, as can be the case, for example, when used in the field ofcatalysis, especially in exhaust systems, they still continue to have aspecific surface area which is quite adequate. Thus, heated to 800° C.,the compositions according to the invention retain a specific surfacearea which is at least 30 m² /g, preferably at least 40 m² /g, and evenmore preferably at least 50 m² /g. When these compositions are heated to900° C., the surface areas are retained at values of at least 20 m² /g,preferably at least 30 m² /g.

In sum, the compositions according to the invention have, at the levelof their specific surface areas, a very good thermal stability.

The presence of the elements cerium and zirconium (and optionallyyttrium) in the compositions of the invention is evidenced by simplechemical analyses, although conventional X-ray diffraction analysesindicate the form in which they exist.

As indicated above, the aforesaid elements are present in thecompositions of the invention in a combined form essentially, andpreferably totally, of the solid solution or mixed oxide type. The X-raydiffraction spectra of these compositions in fact reveal the existenceof only a single identifiable phase (absence of detectable interferingsecondary phase) and which corresponds in actual fact to that of a cericoxide crystallized in the cubic habit and of which the lattice unitparameters are shifted more or less in comparison with a pure cericoxide, thus evidencing the incorporation of zirconium (optionally ofyttrium) in the crystalline lattice of the cerium oxide, and thereforethe production of a true solid solution.

The monophasic mixed oxides according to the invention overallcorrespond to the general formula Ce_(x) Zr_(1-x) O₂ in which x canrange from 0.4 to 1, this latter value being excluded. Advantageously, xis higher than 0.5. More particularly, x may range from 0.4 to 0.9 andeven more preferably from 0.5 to 0.9.

It will thus be seen that the solid solutions of high specific surfacearea consistent with the invention can vary over a very wide range ofcomposition. The upper content limit of zirconium in the composition isindeed only dictated by the sole limit of solubility of this species inthe cerium oxide.

In all instances, and even in particular at significant concentrationsof zirconium (especially higher than 10 atom %), the compositionsaccording to the invention, other than having very high and stablespecific surface areas, continue to be present in a form which isperfectly monophasic and of the cubic CeO₂ type.

For purposes of indicating the significance of the formula given above,it should be noted that the formula Ce₀.7 Zr₀.3 O₂ will then correspondin aggregate to a mixed oxide composition consistent with the inventioncontaining, for example, 30 atom % of zirconium in solid solution in thecerium oxide.

The process for the preparation of the compositions according to theinvention will now be more fully described.

As indicated above, the first stage of the process of the inventionentails preparing a mixture, in aqueous phase, containing at least onesoluble cerium compound and at least one soluble zirconium compound and,optionally, at least one soluble yttrium compound. The mixture can beobtained, equally as well, either from compounds initially in the solidstate which will thereafter be introduced into a base stock of water, ormore directly from solutions of these compounds and then the mixing, inany order, of said solutions.

Exemplary cerium compounds soluble in water include the salts of ceriumIV such as the nitrates or ceric ammonium nitrates, which areparticularly well suited. Preferably, ceric nitrate is used. Thesolution of cerium IV salts can contain, without disadvantage, cerium inthe cerous state, but it is desirable that it contains at least 85% ofcerium IV. An aqueous solution of ceric nitrate can be obtained, forexample, by reacting nitric acid with a hydrated ceric oxide prepared inconventional manner via reaction of a solution of a cerous salt, forexample cerous carbonate, with a solution of ammonia in the presence ofhydrogen peroxide. Preferably, a solution of ceric nitrate is employed,obtained by electrolytic oxidation of a solution of cerous nitrate suchas described in FR-A-2 570 087.

It will also be appreciated that the aqueous solution of salts of ceriumIV can have a certain initial free acidity, for example a normalityranging from 0.1 to 4N. According to the present invention, it ispossible to employ an initial solution of salts of cerium IV effectivelyhaving a certain free acidity as indicated above as a solution whichwill have been neutralized beforehand in a more or less extensive mannerby addition of a base, such as, for example, a solution of ammonia oreven of alkali metal hydroxides (sodium, potassium, etc.), butpreferably a solution of ammonia, such as to limit this acidity. It isthen possible, in this latter instance, to define in a practical mannera ratio of neutralization (r) of the initial solution of cerium by thefollowing equation: ##EQU1## in which n1 represents the total number ofmoles of Ce IV present in the solution after neutralization; n2represents the number of moles of OH⁻ ions effectively required toneutralize the initial free acidity contributed by the aqueous solutionof salt of cerium IV; and n3 represents the total number of moles of OH⁻ions contributed by the addition of the base. When "neutralization" isin fact carried out, in all instances a quantity of base is used whichnecessarily must be less than the quantity of base which would berequired for total precipitation of the hydroxide species Ce(OH)₄ (r=4).In practice, this is thus limited to ratios of neutralization notexceeding 1, and more preferably not exceeding 0.5.

Exemplary soluble compounds of zirconium include the salts of thezirconium sulfate, zirconyl nitrate or, moreover, zirconyl chloridetype. Zirconyl nitrate is particularly well suited.

Finally, when it is desired to obtain a final composition alsocontaining yttrium in solid solution, soluble compounds of yttrium areused such as the nitrates, acetates or halides, in particular chlorides,for example.

It will be appreciated that the cerium, zirconium and optionally yttriumcompounds indicated above are illustrative only.

The amounts of cerium, zirconium and optionally yttrium present in themixture must correspond to the stoichiometric proportions required toprovide the desired final composition.

The initial mixture thus being formulated, in the second stage of theprocess of the invention (stage (ii)) it is heated or thermally treated.

The temperature at which this thermal treatment, also calledthermohydrolysis, is conducted advantageously ranges from 80° C. to thecritical temperature of the reaction mixture. More particularly, it canbe at least 120° C. By way of example, this temperature advantageouslyranges from 80° to 350° C. preferably from 90° to 200° C. and morepreferably from 120° to 200° C.

This treatment can be conducted under the temperature conditionsselected, either under normal atmospheric pressure, or under pressuresuch as, for example, the saturation vapor pressure corresponding to thetemperature of the thermal treatment. When the treatment temperature isselected to be higher than the reflux temperature of the reactionmixture (namely, generally higher than 100° C.) for example selectedfrom 150° to 350° C., the operation is then conducted by introducing theaqueous mixture containing the aforementioned species in an enclosedspace (closed reactor, typically designated an autoclave), the necessarypressure then results only from the sole heating of the reaction mixture(autogenous pressure). Under the temperature conditions indicated above,and in aqueous media, it can thus be specified, by way of illustration,that the pressure in the closed reactor ranges from a value of greaterthan 1 bar (10⁵ Pa) to 165 bar (165.10⁵ Pa), preferably from 5 bar(5.10⁵ Pa) to 165 bar (165.10⁵ Pa). It is of course also possible toexert an external pressure which is added to that existing following theheating.

The heating can be conducted either under an air atmosphere, or under aninert gaseous atmosphere, preferably nitrogen.

The duration of the treatment is not critical, and can thus vary overwide limits, for example from 1 to 48 hours, preferably from 2 to 24hours. In the same manner, the increase in temperature is carried out ata rate which is not critical, and the fixed reaction temperature canthus be achieved by heating the mixture, for example, for from 30minutes to 4 hours.

During the thermal treatment, the pH of the reaction medium typicallyranges from about 0 to 2 at the beginning of the treatment to about 0 to3 upon completion thereof.

At the end of the stage (ii) of heating, a solid product is recoveredwhich can be separated from the reaction medium by all conventionalsolid/liquid separation techniques such as, for example, filtration,separation, draining or centrifugation.

If necessary, to complete preparation of the product, a base such as,for example, an aqueous solution of ammonia can be introduced, directlyafter the heating stage, into the reaction medium. This stage thuspermits the recovery yields of the obtained species to be increased.

It too will be appreciated that it is of course possible to repeat oneor more times, in an identical or different fashion, a heating/reactionstage such as described above, by then employing, for example, cycles ofthermal treatments.

The product as it is recovered can then be subjected to washings, whichare then preferably carried out using an aqueous solution of ammonia. Toeliminate residual water, the washed product can, if appropriate, lastlybe dried, for example in air, at a temperature which can range from 80°to 300° C., preferably from 100° to 150° C., the drying being continueduntil a constant weighing is attained.

In a last stage of the process according to the invention (stage (iv)),which is not obligatory, the recovered product, after washing and/ordrying if appropriate, can then be calcined. This calcination enablesthe crystallinity of the formed solid solution phase to be developed. Itcan also be adjusted and/or selected as a function of the subsequentintended temperature of use reserved for the composition according tothe invention, taking account of the fact that the specific surface areaof the product is so much the lower because the temperature ofcalcination employed is higher. Such a calcination is generally carriedout in air, but a calcination conducted, for example, under an inert gasis obviously also within the scope of this invention.

As emphasized above, solid solutions can thus be prepared by employingexceptionally low synthesis temperatures, on the order of 100° C., thesesolid solutions then having the highest specific surface areas. Also, inactual practice, when a final calcination stage is employed, thecalcination temperature is generally limited to a range of values offrom 200° to 1,000° C., preferably from 400° to 800° C. Even aftercalcinations at high temperatures, i.e., in particular temperatureshigher than those which are strictly necessary for clearly evidencingvia X-ray analysis the formation of the desired solid solution, thecompositions according to the invention retain specific surface areaswhich as wholly acceptable.

Thus, the remarkably high specific surface areas of the novelcompositions according to the invention permit them to be used for verynumerous applications. They are particularly well suited for catalysisapplications, as catalysts and/or as catalyst supports. Notably, theycan be employed as catalysts or catalyst supports for carrying out awide variety of reactions such as, for example, dehydration,hydrosulfuration, hydrodenitrification, desulfuration,hydrodesulfuration, dehydrohalogenation, reforming, vapor-reforming,cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization,dismutation, oxychlorination, dehydrocyclization of hydrocarbons orother organic compounds, oxidation and/or reduction reactions, Clausreaction, the treatment of exhaust gases from internal combustionengines, demetallation, methanation and shift conversion.

Nonetheless, one of the most important applications for the mixed oxidecompositions according to the invention, as emphasized above, is theiruse as constituents of catalysts intended for the treatment orconversion of exhaust gases emanating from internal combustion engines.For this application, the mixed oxide compositions of the invention aregenerally admixed with aluminum before or after impregnation bycatalytically active elements, such as precious metals. Such mixturesare then either shaped to form catalysts, for example in the form ofbeads, or used to form a lining of a refractory body such as a ceramicor metallic monolith, this lining per se being well known to this art asa "washcoat."

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

This example illustrates the preparation of a mixed oxide of cerium andzirconium according to the invention, of formula Ce₀.8 Zr₀.2 O₂.

In the stoichiometric proportions required to obtain the mixed oxideindicated above, a first solution of zirconyl nitrate ZrO(NO₃)₂.2H₂ Owas mixed with a second solution of ceric nitrate, of which the initialfree acidity had been first neutralized by addition of ammonia until aratio of neutralization r (defined above) equal to 0.5 had beenattained.

The mixture thus obtained was then placed in an autoclave (PARRautoclave) and thermally treated at 160° C. for 4 hours.

At the end of this treatment, the product obtained was recovered byfiltration, then washed with a 2M solution of ammonia, and finally driedovernight in an oven at 80° C.

The product was lastly subjected to a calcination stage in air at 400°C. for 6 hours.

The BET specific surface area of the product thus calcined was then 153m² /g.

The X-ray diffraction diagram of this product is shown in FIG. 1, curvea.

In comparison with the X-ray diffraction spectrum of a pure ceric oxideprepared under the same conditions as above, but in the absence ofzirconium (FIG. 1, curve b), a clear displacement in the position of thediffraction peaks towards large angles is observed for the product ofthe invention. This evidences the incorporation of the zirconium intothe crystalline lattice of the cerium oxide. In addition, only the peakscorresponding to the cubic phase of the type CeO₂ can be detected in theX-ray diffraction diagram of the product, it not being possible todetect interfering secondary phase.

This point is moreover confirmed when a second calcination, this timeeffected at 700° C. for 2 hours, was carried out on the product of theinvention. Indeed, the X-ray diffraction spectrum of the product thusobtained (FIG. 2) always evidences only a single crystalline phase. Inaddition, under these conditions, the degree of crystallization of theproduct was sufficient for it to be possible to measure the lattice unitparameter of the cubic phase of the type CeO₂ obtained. The value ofthis lattice unit parameter measured was then 5.36 Å (0.536 nm). It wasin very good agreement with the value which can be estimated from theaforesaid publication of E. Tani et al for a mixed oxide of compositionCe₀.8 Zr₀.2 O₂ (cf. "evaluation of the evolution of the latticeparameter in the system CeO₂ --ZrO₂ as a function of the rate ofsubstitution of the cerium atoms by those of zirconium") and which wasthen also 5.36 Å (0.536 nm).

These results thus clearly demonstrate that, according to the presentinvention, a phase Ce₀.8 Zr₀.2 O₂ of the solid solution type was presentat most from 400° C., which then had a BET specific surface area of 153m² /g.

EXAMPLE 2

This example illustrates the preparation of a mixed oxide of cerium,zirconium and yttrium according to the invention, of formula Ce₀.65Zr₀.30 Y₀.05 O₂.

In the stoichiometric proportions required to obtain the mixed oxideindicated above, (a) a solution of ceric nitrate having a free acidityof 0.62N, (b) a solution of zirconyl nitrate and (c) a solution ofyttrium nitrate were mixed with shaking.

The mixture was then treated thermally at 150° C. for 4 hours in anautoclave, with constant mechanical stirring of the medium.

At the end of this treatment, a solution of ammonia was introduced intothe resulting suspension, such as to increase the pH to 9.5, the entireassembly then being shaken for 30 minutes for homogenization.

A product was the recovered by filtration and was drained and thenresuspended in water. This suspension was next heated to 100° C. for 1hour.

The product was filtered again and then dried in an oven at 120° C.

The dried product was lastly calcined in air at three differenttemperatures for 6 hours, namely, 400° C., 800° C. and 900° C.; thespecific surface areas of the products obtained then being,respectively, 157 m² /g, 53 m² /g and 39 m² /g.

The X-ray diffraction diagram of the product obtained after calcinationat 800° C. for 6 hours is shown in FIG. 3.

The lattice unit parameter measured was then 5.33 Å (0.533 nm), whichagain corresponded to the theoretical lattice unit parameter which canbe estimated from the aforesaid publication of Tani et al.

EXAMPLE 3

This example illustrates the preparation of a mixed oxide of cerium andzirconium according to the invention, of formula Ce₀.83 Zr₀.17 O₂.

In the stoichiometric proportions required to obtain the mixed oxideindicated above, a solution of zirconyl nitrate was mixed with asolution of ceric nitrate whose initial free acidity had previously beenneutralized with ammonia such as to provide a ratio of neutralization requal to 0.

The procedure then followed was rigorously identical to that of Example2.

The specific surface areas, at different calcination temperatures (6hours), of the final products obtained were the following:

(i) 400° C.: 118 m² /g

(ii) 800° C.: 35 m² /g

(iii) 900° C.: 26 m² /g

The product calcined at 800° C. had a cubic solid solution phase, ofwhich the measured lattice unit parameter was 5.38 Å (0.538 nm).

EXAMPLE 4

This example illustrates the preparation of a mixed oxide of identicalformula to that of Example 2 (Ce₀.65 Zr₀.30 Y₀.05 O₂), but following adifferent procedure than that of Example 2 (not employing an autoclavingstage at 150° C.

The aqueous mixture prepared in Example 2 was subjected to the followingcycle of thermal treatment (heating):

(1) first heating at 100° C. for 2 h, 30 min;

(2) cooling of the mixture to 45° C. in 60 minutes;

(3) addition of ammonia to adjust the pH of the medium to a value offrom 9 to 9.5;

(4) second heating at 100° C. for 1 hour;

(5) cooling to ambient temperature.

The product was then recovered by filtration and dried in an oven at120° C.

The specific surface areas, at different calcination temperatures (6hours), of the final products obtained were the following:

(i) 400° C.: 112 m² /g

(ii) 800° C.: 54 m² /g

(iii) 900° C.: 33 m² /g

The product obtained after calcination at 800° C. had a cubic solidsolution phase, of which the lattice unit parameter measured was 5.33 Å(0.533 nm).

The X-ray diffraction spectrum of this product is shown in FIG. 4.

EXAMPLE 5

This example illustrates the preparation of a mixed oxide of identicalformula as that of Example 3 (Ce₀.83 Zr₀.17 O₂), but according to adifferent procedure (no employment of an autoclaving stage at 150° C.).

In the stoichiometric proportions required to obtain the desired mixedoxide, a solution of zirconyl nitrate was mixed with a solution ofcerium nitrate having a free acidity of 0.62N.

The mixture thus prepared was then treated rigorously following theprocedure of Example 4.

The specific surface areas, at different temperatures of calcination (6hours), of the final products obtained were the following:

(i) 400° C.: 151 m² /g

(ii) 800° C.: 57 m² /g

(iii) 900° C.: 33 m² /g

The product obtained after calcination at 800° C. was a cubic solidsolution phase, of which the measured lattice unit parameter was 5.36 Å(0.536 nm).

The X-ray diffraction spectrum of this product is shown in FIG. 5.

EXAMPLE 6 (Comparative)

In this example, it was attempted to prepare a mixed oxide of identicalformula as that of Example 2 (Ce₀.65 Zr₀.30 Y₀.05 O₂), but employing aconventional route by impregnation.

In the stoichiometric proportions required to obtain a mixed oxide ofthe above formula, a ceric oxide CeO₂ of commercial grade having aspecific surface area of 250 m₂ /g (product marketed by RHONE-POULENC)was impregnated by means of a first solution of zirconyl nitrate. Theproduct thus impregnated then was dried at 110° C. in air, next thedried product was impregnated by means of a second solution of yttriumacetate Y(C₂ H₃ O₂)₃.4H₂ O, and the product thus impregnated was driedagain in air at 110° C.

The ceric oxide thus impregnated with zirconium and yttrium was thencalcined for 6 hours at 900° C.

The X-ray diffraction diagram of the product then obtained is shown inFIG. 6.

Analysis of the spectrum evidences the existence of diffraction peaksattributable to the pressure of a zirconium oxide phase ZrO₂, thusdemonstrating that it had not been possible to obtain a pure phase ofthe mixed oxide type.

The specific surface area of this product was 20 m² /g.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A cerium/zirconium mixed oxide presenting a pure, monophasic CeO₂ cubic crystalline habit of the type CeO₂ and having a specific surface area of at least 30 m² /g after calcination at 800° C. for 6 hours.
 2. The cerium/zirconium mixed oxide as defined by claim 1, having a specific surface area of at least 40 m² /g after calcination at 800° C. for 6 hours.
 3. The cerium/zirconium mixed oxide as defined by claim 1, having a specific surface area of at least 50 m² /g after calcination at 800° C. for 6 hours.
 4. The cerium/zirconium mixed oxide as defined by claim 1, having a specific surface area of at least 20 m² /g after calcination at 900° C. for 6 hours.
 5. The cerium/zirconium mixed oxide as defined by claim 4 having a specific surface area of at least 30 m² /g after calcination at 900° C. for 6 hours.
 6. The cerium/zirconium mixed oxide as defined by claim 1, comprising a solid solution thereof.
 7. The cerium/zirconium mixed oxide as defined by claim 6, further comprising yttrium in the solid solution.
 8. Particulates of a monophasic solid solution of a cerium/zirconium mixed oxide having the formula Ce_(x) Zr_(1-x) O₂, in which x is a number ranging from 0.5 to less than 1 and retaining a specific surface area of at least 50 m² /g when heated at 800° C. for six hours, wherein the zirconium is incorporated in the crystalline lattice of the cerium oxide.
 9. The particulates of a cerium/zirconium mixed oxide as defined by claim 8, wherein x ranges from 0.5 to 0.9.
 10. The particulates of a cerium/zirconium mixed oxide as defined by claim 8, further comprising yttrium in the solid solution.
 11. A process for the preparation of the cerium/zirconium mixed oxide as defined by claim 1, comprising thermohydrolysis of an aqueous formulation of water-soluble compounds of cerium and zirconium, said compounds of cerium and zirconium being present in stoichiometric amounts, and thence recovering the reaction product thus produced.
 12. The process as defined by claim 11, further comprising calcining said reaction product thus recovered.
 13. The process as defined by claim 11, said compounds of cerium and zirconium comprising salts thereof.
 14. The process as defined by claim 13, said cerium compound comprising ceric nitrate or ceric ammonium nitrate.
 15. The process as defined by claim 14, said zirconium compound comprising zirconium sulfate, zirconyl nitrate, zirconyl chloride, or admixture thereof.
 16. The process as defined by claim 11, comprising thermally treating/reacting at a temperature ranging from 80° C. to the critical temperature of the reaction mixture.
 17. The process as defined by claim 11, further comprising adding a base to the medium of reaction.
 18. The process as defined by claim 17, said base comprising an aqueous ammonia solution.
 19. The process as defined by claim 12, comprising calcining at a temperature ranging from 200° to 1,000° C.
 20. A catalyst/catalyst support comprising the cerium/zirconium mixed oxide as defined by claim
 1. 21. A catalyst as defined by claim 20, the support therefor comprising aluminum values.
 22. A catalyst as defined by claim 20, comprising a monolith.
 23. A catalyst as defined by claim 21, comprising a monolith.
 24. A cerium/zirconium mixed oxide having the formula Ce_(x) Zr_(1-x) O₂ in which x is greater than 0.5 and less than 1, said mixed oxide having a specific surface area of at least 80 m² /g and retaining a specific surface area of at least 50 m² /g when heated at 800° C. for six hours.
 25. A cerium/zirconium mixed oxide presenting a pure, monophasic CeO₂ cubic crystalline habit of the type CeO₂, having the formula Ce_(x) Zr_(1-x) O₂ in which x is a number ranging from 0.5 to 0.8 and having a specific surface area of at least 30 m² /g after calcination at 800° C. for 6 hours.
 26. The mixed oxide as defined by claim 25, wherein x ranges from 0.5 to 0.7.
 27. The mixed oxide as defined by claim 25, having a specific surface area of at least 40 m² /g after calcination at 800° C. for 6 hours.
 28. The mixed oxide as defined by claim 25, having a specific surface area of at least 50 m² /g after calcination at 800° C. for 6 hours.
 29. The mixed oxide as defined by claim 25, having a specific surface area of at least 20 m² /g after calcination at 900° C. for 6 hours.
 30. The mixed oxide as defined by claim 25, having a specific surface area of at least 30 m² /g after calcination at 900° C. for 6 hours.
 31. The mixed oxide as defined by claim 25, comprising a solid solution.
 32. The mixed oxide as defined by claim 31, further comprising yttrium in the solid solution.
 33. A composition based on cerium and zirconium which after heating at a temperature between 400° and 800° C., presents a pure monophasic CeO₂ cubic crystalline habit and wherein zirconium is incorporated in the crystalline habit of the cerium oxide.
 34. A composition according to claim 33 which after heating at a temperature between 400° and 800° C., has the formula Ce_(x) Zr_(1-x) O₂, in which x is a number ranging from 0.5 to less than
 1. 35. A composition according to claim 33 and having a specific surface area of at least 30 m² /g after calcination at 800° C. for 6 hours.
 36. A composition according to claim 33 and having a specific surface area of at least 40 m² /g after calcination at 800° C. for 6 hours.
 37. A composition according to claim 33 and having a specific surface area of at least 50 m² /g after calcination at 800° C. for 6 hours.
 38. A composition according to claim 33 and having a specific surface area of at least 20 m² /g after calcination at 900° C. for 6 hours.
 39. A composition according to claim 33 and having a specific surface area of at least 30 m² /g after calcination at 900° C. for 6 hours.
 40. The process as defined by claim 8, wherein the thermohydrolysis is carried out at a temperature from 80° to 350° C.
 41. The process as defined by claim 8, wherein the thermohydrolysis is carried out at a temperature from 90° to 200° C.
 42. The process as defined by claim 8, wherein the aqueous formulation has a pH of 0 to
 3. 